1. Field of the Invention
The present invention relates to an optical head device, and more particularly to a birefringent diffraction grating polarizer, a polarizing hologram element and a optical head device using such polarizer or hologram element.
2. Description of the Related Art
The polarizer, especially, the polarizing beam splitter is an element with which a predetermined polarized light beam is obtained by causing the light propagation directions changed, in the polarized beams, to be orthogonal to each other. Conventional examples of the polarizing beam splitters often used include a Glan-Thompson prism or a Rochon prism. In one example, the optical path is separated by utilization of differences in the transmission or reflection due to the polarization at cemented surfaces of crystal compounds having large birefringences. In another example, a dielectric multi-layer film is provided at the cemented surface of a compound prism type beam splitter and, by utilization of differences in the interference in the dielectric multi-layer film due to the polarization, the light is caused to be reflected or transmitted. The drawbacks in such elements are that the size thereof is large, the productivity thereof is low, and the cost thereof is high. Also, the polarizer of a bulk type as above only has polarizing functions so that, when it is used for an optical disk head device, there are difficulties in compositely accommodating other functions such as those for focusing error detection and tracking error detection, and these are obstacles to the scaling down of the optical head device.
A recently developed polarizer in which the productivity is high and the composite functions other than the polarization function are accommodated is a birefringent diffraction grating polarizer which is disclosed in Japanese Patent Application Kokai Publication No. 63-314502. FIG. 1 is a sectional view showing such birefringent diffraction grating polarizer. Where a proton exchange region of benzoic acid is provided on an X-plate or a Y-plate of a lithium niobate substrate 1 which is a birefringent crystal, it is noted that, with respect to a wavelength of, for example, 0.78 .mu.m which is generally used in the optical disk device, the index of refraction for ordinary light which is polarized parallel to the crystal optical axis of the substrate increases by about 0.12 and the index of refraction for ordinary light which is polarized perpendicular to the optical axis decreases by about 0.04. Thus, the grating in which the exchange region 2 with the proton and the non-exchange region without the proton are arranged periodically functions as a diffraction grating. When this grating is configured such that a phase compensation film 3 of an appropriate thickness is formed on the exchange region 2 for mutually canceling the phase difference between the ordinary light passing through the exchange region 2 and the ordinary light passing through the non-exchange region, the grating does not function as a diffraction grating with respect to the ordinary light, whereby such ordinary light is allowed to be transmitted without being diffracted. That is, this grating is seen simply as a transparent plate. Where the phase difference with respect to the extraordinary light is .pi. when the depth of the exchange region 2 is changed while maintaining the mutual phase difference cancellation conditions with respect to the ordinary light and, moreover, the width of the proton exchange region 2 and that of the non-exchange region are the same with each other, the extraordinary light is completely diffracted. The relationship of these phases may be expressed by the following equations: EQU {.DELTA.n.sub.e .multidot.T.sub.p +(n.sub.d -n.sub.out).multidot.T.sub.d }.multidot.2 .pi./.lambda.=.pi. EQU {.DELTA.n.sub.o .multidot.T.sub.p +(n.sub.d -n.sub.out).multidot. T.sub.d }.multidot.2 .pi./.lambda.=0
wherein n.sub.d and T.sub.d are respectively an index of refraction and a thickness of the phase compensation film 3, T.sub.p is a depth of the proton exchange region 2, .DELTA.n.sub.e and .DELTA.n.sub.o are amounts of changes in the index of refraction in the extraordinary light and the ordinary light at the proton exchange region 2, and n.sub.out is an index of refraction outside the hologram substrate, that is, n.sub.out =1 in an air layer. Also, .lambda. is a wavelength of light.
Also, it is possible to realize the polarization function by the arrangement wherein the proton exchange region 2 and the phase compensation film 3 are disposed alternately, the phase difference between the light passing through the proton exchange region 2 and the light passing through the phase compensation film 3 of dielectric is made zero with respect to the extraordinary light, the phase difference therebetween is made .pi. with respect to the ordinary light, and the width of the proton exchange region 2 and that of the phase compensation film 3 are made the same with each other. In this case, the extraordinary light is transmitted without being diffracted and the ordinary light is completely diffracted.
The disclosure of examples in which the above birefringent diffraction grating polarizer is used in the optical head device is found in Japanese Patent Application Kokai Publication No. 3-29137 and Japanese Patent Application Kokai Publication No. 3-29129 which respectively disclose hologram elements. While these hologram elements are configured as shown in a sectional view in FIG. 1, the grating patterns are constituted by a plurality of grating regions having different diffraction directions as shown in FIG. 2 for purposes of detecting focusing error signals and tracking error signals in the optical head. In similarly arranged grating patterns, the grating regions are constituted by divided patterns as shown in FIG. 3.
FIG. 4 diagrammatically shows an optical head device for use in a video disk, a write once read many type optical disk and a rewritable type phase-change optical disk as disclosed in Japanese Patent Application Kokai Publication No. 3-29129. The light emitted from a semiconductor laser 10 is incident on a hologram element 16 as ordinary light, transmitted without being diffracted, passes through a collimating lens 11, a 1/4 wavelength plate 13 and an objective lens 12, and is converged on an optical disk 14. The returning light from the optical disk 14 is incident again on the hologram element 16 after passing through the common path in reverse. The polarizing surface of the returning light is rotated 90 degrees with respect to that of the original polarized light by the 1/4 wavelength plate 13 so that such light is incident on the hologram element 16 as extraordinary light and is completely diffracted, and the +1 order diffracted light 50 and the -1 order diffracted light 51 are received respectively by a first photodetector 30 and a second photodetector 31.
FIG. 5 shows in a plane view the first photodetector 30 and the second photodetector 31 with the semiconductor laser disposed between them and shows a state of the diffracted light beams incident respectively on the photodetectors. The diffracted light beams from a region 5(A) and the diffracted light beams from a region 6(B) in the hologram (shown in FIG. 2) converge respectively at a converging point 40 and a converging point 41 shown in FIG. 5. A dividing line 9 on the hologram pattern in FIG. 2 functions as a knife edge and, by a double-knife-edge method, a focusing error signal is detected from these diffracted light beams. A tracking error is detected by a push-pull method based on the difference between the diffracted light beams from the region 7(C) and the diffracted light beams from the region 8(D) which converge respectively at the points 42 and 43 shown in FIG. 5. A recording signal is detected based on the light received from the second photodetector 31 or on the sum of the amounts of light received from both the first photodetector 30 and the second photodetector 31. Also, by arranging to receive higher order diffracted beams, it is possible to have the intensity of the signal further increased.
FIG. 6 shows an optical head device for a magneto-optical disk as disclosed in Japanese Patent Application Kokai Publication No. 3-29137. The light emitted from a semiconductor laser 10 passes through a collimating lens 11, a polarizing beam splitter 18 and an objective lens 12 and then is converged on a magneto-optical disk 15. Following a reverse path, the returning light from the magneto-optical disk 15 is separated to outside an optical axis by the polarizing beam splitter 18, is caused to be converged by a lens 19, whereby the extraordinary light component is diffracted by a hologram element 17, and is received by a first photodetector 32, a second photodetector 33 and a third photodetector 34. The polarizing beam splitter 18 has polarization characteristics which cause the p-polarized light from the semiconductor laser 10 to be partially reflected and residually transmitted and also cause the s-polarized light orthogonal thereto, that is, slight amounts of polarized light occurring by Kerr effect at the reflection on the magneto-optical disk 15, to be completely reflected. The recording signal is detected by a differential detection method from the 0 order diffracted light and the + 1 order diffracted light separated by polarization due to the polarizing function of the hologram element 17. Where the detection relies on the difference between the sum of +1 order diffracted light 52 and -1 order diffracted light 54 and the 0 order diffracted light 53, the amounts of light become balanced and the recording signal can be detected with small noise when the direction of the returning light is at an angle of 42 degrees with respect to the crystal optical axis of the hologram element. When the detection relies on the difference between the -1 order diffraction light 54 and the 0 order diffracted light 53, the angle may be set to about 32 degrees. FIG. 7 is a diagram for illustrating a state of the diffracted light beams where they are incident on the photodetectors. Focusing and tracking error signals are detected In the same way as in the case of FIG. 5 by using the +1 order diffracted light 52 which is incident on the first photodetector 32.
The above described birefringent diffraction grating polarizer is a light transmission type so that the process of forming a grating layer requires a long time because it is necessary to make the proton exchange region deep and also it is necessary to make the phase compensation film thick. Also, if this element is used in the conventional optical head device described above, it will be difficult to make the device compact.
FIG. 8 shows a conventional optical head device for use in a magneto-optical disk drive. Emitted light 82 from a semiconductor laser 81 is converted to collimated light 84 by a collimating lens 83 and, after being transmitted through a beam splitter 85 and totally reflected at a total reflection prism 86 with the light path being bent 90 degrees, is converged on an optical disk surface 88 by a convergent lens 87. The reflected light returning from the optical disk follows the common path in reverse and is reflected at the beam splitter 85. The reflected light changes its polarization direction 90 degrees at a 1/2 wavelength plate 89, is converted to converging light by a lens 90 and is divided by a polarizing beam splitter 93 into the polarized transmission light 91 and the reflection light 92 the polarization directions of which are orthogonal to each other. The transmission light 91 is incident on a half-split photodetector 94, and a tracking error signal is obtained based on a differential signal from photodetector elements 95 and 96 by a push-pull method. On the other hand, the reflection light 92 forms an astigmatic wave surface due to a cylindrical lens 97, and a focusing error signal is obtained through a quarter-split photodetector 98 using an astigmatic method. That is, when the output voltages of photodetector elements 100, 101, 102 and 103 are assumed to be respectively V.sub.(100), V.sub.(101), V.sub.(102) and V.sub.(103), the focusing error signal can be obtained by V.sub.(100) +V.sub.(101) -V.sub.(102) -V.sub.(103). The read-out signal is obtained as a differential signal based on intensity differences between the polarized light beams which are split by the polarization beam splitter and the polarization directions of which are orthogonal with each other. Thus, assuming that the output voltages of the photodetector elements 95 and 96 are respectively V.sub.(95) and V.sub.(96), the read-out signal will be V.sub.(95) +V.sub.(96) -V.sub.(100) -V.sub.(101) -V.sub.(102) -V.sub.(103).
For the magneto-optical head device utilizing the polarizing hologram, reference is made to Japanese Patent Application Kokai Publication No. 3-29137 which has already been identified above and which discloses the arrangement as shown in FIG. 6. That arrangement employs the polarizing hologram formed by providing ion exchange on a lithium niobate crystal and this enables to reduce the optical system after the 1/2 wavelength plate 89 in FIG. 8 to three elements, namely, the polarizing hologram, lens and photodetectors.
However, the above conventional optical head device 1s still large in size, and a device in practical use is larger than about 40.times.40.times.30 mm and weighs heavy accordingly. This has been a barrier in enhancing high speed tracking access and scaling down of the optical disk device as a whole.